Gas monitoring devices, such as gas spectrometers, are utilized in a wide variety of industrial and medical applications to monitor the presence and concentration of one or more predefined components in a gas sample. Typically, light of a known spectral content is directed through a gas sample and the intensity of the transmitted light at a number of different center-wavelengths is detected. By utilizing known light absorption characteristics of the predefined gas components at the center-wavelengths, the detected light intensities provide a basis to determine, via statistical processing, the concentrations of the predefined components. As will be appreciated, it is important that the initial calibration conditions of the spectrometer be maintained in order to accurately relate the measured light intensities to gas component concentrations.
This is particularly true in respiratory gas spectrometers used to measure the concentration of carbon dioxide and/or oxygen and one or more anesthetic agents, such as nitrous oxide, halothane, enflurane, isoflurane, sevoflurane and desflurane, in a respiratory gas stream. In such applications, a separate sample stream is typically drawn from the patient respiratory gas assembly and directed into a sample chamber that is positioned on the optical path between the light source and the detector. To maintain high accuracy, any significant absorbers of light at the center-wavelengths of interest that are on the optical pathway between the light source and detector should be accounted for during calibration and the related calibration conditions should be maintained during use.
Given such calibration considerations, the optical pathway(s) utilized in respiratory gas spectrometers may be disposed within a sealed housing. Because even a small variation in the concentration of absorbers of light, such as carbon dioxide, may require recalibration, it is important that the housing be adequately sealed from sources of gaseous contaminants. In this regard, it should be noted that, given the responsivity needs of respiratory gas spectrometers, it has been found desirable to utilize light sources operating at relatively high temperatures (e.g., temperatures exceeding 900.degree. C.). In particular, such light sources may comprise one or more ceramic elements for actual radiation transmission. While such sources provide high responsivity they also generate a large amount of heat which can present challenges for maintaining a sealed environment.
More particularly, the present inventors have recognized that the use of a high temperature light source may affect the sealing of a gas spectrometer due to, for example, outgassing from or leakage through/around sealing components (e.g., gaskets, o-rings, sealing compounds, and the like) and leakage due to differing thermal expansion/contraction rates of adjoined components. Further, it has been recognized that excessive heat transfer from a high temperature light source may adversely impact the operation/calibration of electrical componentry used in a spectrometer, including in particular the radiation detectors employed therein.